Interfacial kinetics of synergistic extraction of samarium(iii) studied by micro-two-phase sheath flow/fluorescence microscopy

Kinetic mechanisms of the interfacial complexation in the synergistic extraction of Sm(III) with 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (2-thenoyltrifluoroacetone, Htta) and tri-n-octylphosphine oxide (topo) in toluene were investigated by means of the micro-two-phase sheath flow/fluorescence microscopy. This technique was used not only for the kinetic measurement of the interfacial complex formation, but also for the fluorescence lifetime measurement of the interfacial complex. By flowing a toluene solution of Htta and topo into an aqueous solution flow including Sm(III) ion, the formation of the fluorescent complex at the interface of the sheath flow system was observed along the flow. From the fluorescence decay image along the inner flow just after the excitation point, the fluorescence lifetime of the formed Sm(III) complex was measured at various reaction times. The observed lifetimes indicated the formation of Sm(tta)3(topo)(H2O)2 as the reaction intermediate in the initial stage of the complexation as well as more stable Sm(tta)3(H2O)3 and Sm(tta)3(topo)2 complexes at the toluene/water interface. The complexation rate was analyzed by the digital simulation method and the rate constant for the formation of Sm(tta)2+ was obtained as 2.4 × 107 mol−1 dm3 s−1, which suggested the Eigen mechanism for the complexation of Sm(III) at the liquid/liquid interface.